My son, almost 7, got a DS for Christmas a year ago.

The giant hits on DS in our house:

Lego anything- we have Star Wars, Batman and Indiana Jones
Pokemon anything- we have Diamond, Pearl and Ranger
Mario anything- we have Kirby, Super Mario Bros and another I'm forgetting.

Other favorites: Henry Hatsworth, Drawn to Life and Drawn to Life the Next Chapter.

Busts: Guitar Hero, Chicken Little, Petz:Tigers and Harry Potter